Method of making TA2 O5 thin film by low temperature ozone plasma annealing (oxidation)

ABSTRACT

A method for low temperature annealing (oxidation) of high dielectric constant Ta 2  O 5  thin films uses an ozone enhanced plasma. The films produced are especially applicable to 64 and 256 Mbit DRAM applications. The ozone enhanced plasma annealing process for thin film Ta 2  O 5  reduces the processing temperature to 400° C. and achieves comparable film quality, making the Ta 2  O 5  films more suitable for Ultra-Large Scale Integration (ULSI) applications (storage dielectric for 64 and 256 Megabit DRAMs with stack capacitor structures, etc.) or others that require low temperature processing. This low temperature process is extendable to other high dc and piezoelectric thin films which may have many other applications.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method of producing highdielectric constant materials on a semiconductor substrate and, moreparticularly, to a method of improving the quality of tantalum pentoxide(Ta₂ O₅) films with low temperature annealing.

2. Description of the Prior Art

High dielectric constant (dc) materials are useful as capacitordielectrics of the storage capacitor in dynamic random access memories(DRAMs). High dc Ta₂ O₅ film has been seriously studied as a storagecapacitor dielectric for 64 and 256 Mbit dynamic random access memories(DRAMs), especially for stack capacitor structures. Most of these filmsare deposited at low temperature (i.e., 400°-480° C.) using chemicalvapor deposition (CVD). However, the primary disadvantage of Ta₂ O₅ ascompared to insulators with lower dielectric constants is its fairlyhigh conductivity with substantial current leakage due to eitherdeviation from stoichiometric composition or defective bonding.Therefore, Ta₂ O₅ films require substantially a higher temperature(i.e., 750°-800° C.) annealing cycle to improve the film's electricalproperties. One method is Rapid Thermal Oxidation (RTO). The hightemperatures used in this method may cause oxidation of nearbypolysilicon, degrading the dielectric properties. Another method used isultraviolet (UV) ozone in which samples are exposed to UV light underozone ambient. UV ozone has a long processing time and may also causehigh temperatures locally. Ozone plasma has been studied as a "lowtemperature plasma oxidation process" to grow excellent thin siliconoxide film. However, the use of plasma oxidation to improve other highdielectric constant materials has not been done.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof decreasing the leakage normally associated with CVD deposited Ta₂ O₅films.

It is another object of the present invention to provide shorterprocessing time of Ta₂ O₅ films more suitable with single wafer processintegration.

According to the invention, a method is provided for low temperatureozone plasma annealing (oxidation) of high dielectric constant Ta₂ O₅thin films. The films produced are especially applicable to 64 and 256Mbit DRAM applications.

The ozone plasma annealing process according to the invention for thinfilm Ta₂ O₅ reduces the processing temperature to 400° C. and achievescomparable film quality, making the Ta₂ O₅ films more suitable forUltra-Large Scale Integration (ULSI) applications (as storage dielectricfor 64 and 256 Megabit DRAMs with stack capacitor structures, etc.) orothers that require low temperature processing. This low temperatureprocess is extendable to other high dc and piezoelectric thin filmswhich may have many other applications.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIG. 1 is a flow chart illustrating the annealing process for both RTOand plasma enhanced oxidation;

FIG. 2 is a graph depicting the results of the RTO anneal;

FIG. 3 is a graph depicting the results the enhanced plasma annealaccording to the invention; and

FIG. 4 is a graph of the data from FIGS. 2 and 3 which has beennormalized to the same applied field.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The process of the present invention improves the Ta₂ O₅ film'sproperties, providing low leakage and high electrical breakdown, by aplasma oxidation annealing process with much shorter processing time.Plasma annealing is more effective at lower temperature. This is due tothe more effective oxidation mechanism by ozone plasma as compared withregular UV ozone annealing. In ozone plasma, the ozone concentration ismuch higher due to the continuous recombination and generation of ozoneby ions and radicals in the plasma discharge. Furthermore, the oxygenions and radicals in ozone plasma also assist the oxidation and improveleakage and breakdown of the Ta₂ O₅ film. This makes the ozone plasmaoxidation/annealing process superior to UV ozone annealing.

Referring now to the drawings, and more particularly to FIG. 1, there isshown a flow chart of the process according to a preferred embodiment ofthe invention. A p-type silicon substrate is first prepared by rapidthermal nitridation (10.11Å) in step 10. This rapid thermal nitridationstep is to form a thin nitride film to prevent oxidation of the siliconsubstrate (n- or p-type) during subsequent oxidation annealing steps.Then a thin film of Ta₂ O₅ is applied to the substrate by low pressurechemical vapor deposition (LPCVD) using Ta(OEt)₅ +O₂ at 460° C. in step11. For purposes of comparative tests, two samples were preparedfollowing steps 10 and 11. The next step 12 is an oxidation step;however, one sample was annealed using a rapid thermal oxidation (RTO)process and the second sample was annealed using the plasma enhancedoxidation anneal according to the invention. The RTO anneal was carriedout at 750° C. with 2000 sccm O₂ for 120 seconds. The initial thicknessof the Ta₂ O₅ film was 24.1 nm, and the resulting thickness afteroxidation was 22.6 nm. The enhanced plasma anneal according to theinvention was carried out with a 95W plasma at 400° C. with 2000 sccm O₃+230 sccm O₂ at 5 Torr for five minutes. The initial thickness of theTa₂ O₅ film was 11.4 nm, and the resulting thickness after oxidation was9.65 nm. Next, in step 13, 0.76 mm² aluminum dots were evaporated on thefront sides of the samples with a backside blanket of aluminum with aforming gas anneal at 400° C. for 30 minutes.

FIGS. 2 and 3 respectively show the measured results for the two sets ofsamples with Rapid Thermal Oxidation (RTO) anneal and a plasma ozoneanneal for various aluminum dots using MOS IV measurement. As can beseen, despite the fact that the RTO anneal sample was thicker, theleakage was much worse. This is because the high temperatures of the RTOanneal causes oxidation of nearby silicon (polysilicon), thus degradingthe dielectric properties. At high temperature oxidation the very thinnitrided surface is not effective in preventing the oxidation.

In FIG. 4, the data from both samples is normalized to the same appliedfield (MV/cm). The superiority of plasma ozone anneal is quite clearfrom this graph. Leakage is 10⁻⁴ A/cm² at 3 MV/cm for the plasma ozoneanneal sample.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is as follows:
 1. A method of producing highdielectric constant Ta₂ O₅ thin films comprising the steps of:subjectinga p- or n-type silicon substrate to a rapid thermal nitridation;depositing by low pressure chemical vapor deposition (LPCVD) a thin Ta₂O₅ film on the nitrided substrate; and annealing the Ta₂ O₅ film with alow temperature ozone and oxygen enhanced plasma.
 2. The method recitedin claim 1 wherein the annealing step is performed at approximately 400°C.
 3. The method recited in claim 2 wherein the annealing step isperformed with 2000 sccm O₃ and 230 sccm O₂ at 5 Torr for five minutes.4. The method recited in claim 1 further comprising the step ofevaporating metal dots on the annealed Ta₂ O₅ film to form capacitorstructures.
 5. The method recited in claim 1, wherein the thin Ta₂ O₅film is about 10Å.